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Why does an aircraft take off?

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Why does an aircraft take off?

Old 29th Oct 2012, 17:52
  #41 (permalink)  
 
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So far in this thread we've seen 'experts' declaring that "During descent the weight overcomes lift, but some lift is still required to stop the aircraft just plummeting out of the sky." whilst admonishing others for not understanding the "relationship between, speed, and angle of attack over an airfoil." amongst other such muddled misconceptions.

We've also had a L1011 whose engines "can only produce about 70,000 tons of thrust."

All we need to add to the mix now is a converor belt, followed by a downwind turn and we'll all be falling out of the sky, or more likely, falling out of our chairs laughing.

Keep up the good work, and please, please keep the gems of humour coming!
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Old 29th Oct 2012, 18:33
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Well, perhaps I'm missing something obvious here but I thought an aircraft takes off because it (or its inhabitants) want to go somewhere....
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Old 29th Oct 2012, 18:47
  #43 (permalink)  
 
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The Definitive Answer

Lift and Thrust

It's a long read but will aid your understanding.
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Old 29th Oct 2012, 19:30
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Lord Spandex - Many thanks for your kind reference to this erudite work. The final reference in connection with kite-flying is apposite and I would comment that the better behaved lift demons are bred at sea and arrive here on the west wind, as today when I enjoyed particularly good flying.
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Old 29th Oct 2012, 19:58
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Max lift implies max G-load - for example in maneuvering, like a tight turn, pullup, or stall recovery.

At takeoff you're only pulling maybe 1.1 G.

BUT - because of the low airspeed, CL is probably highest (or close) at takeoff.
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Old 29th Oct 2012, 20:49
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Airmann

I couldn't beleive how many posts it took to get to the right answer on this thread.
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Old 29th Oct 2012, 21:06
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Surely the answer is 'bernoulli's principle'?
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Old 29th Oct 2012, 23:51
  #48 (permalink)  
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The Definitive Answer

.. now that's let the cat out of the bag ...
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Old 30th Oct 2012, 08:53
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or more likely, falling out of our chairs laughing.
I already am!!!
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Old 30th Oct 2012, 08:53
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How about the B52 take off.

It's the curvature of the earth that gets it airborne at heavy weights.
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Old 30th Oct 2012, 09:55
  #51 (permalink)  
 
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is it not a sweety to see all the mental effusions for a question where the basic answer is quite obvious ?

aicrafts have (on good days) wings attached to the hull- and there is a reason for it.

wikipedia helps a lot

Aircraft - Wikipedia, the free encyclopedia

cheers !
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Old 30th Oct 2012, 10:39
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What is the thrust angle relative to the horizontal at take off?

Google suggests that the A380 engines produce a total thrust of 130,000 kg roughly. If the thrust angle was 20 degrees the vertical component would be

130,000 * sin(20) = 44,000Kg

The aircraft max TOW is around 560,000kg so the vertical component of thrust is about 10% of the total force required to lift off.
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Old 30th Oct 2012, 12:23
  #53 (permalink)  
 
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That's the point I made - albeit with a mixup of units for the engines!
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Old 30th Oct 2012, 12:50
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If you have big enough flaps, rotation is unnecessary:

Last edited by barit1; 30th Oct 2012 at 12:51.
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Old 30th Oct 2012, 15:59
  #55 (permalink)  

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In an airliner's flight profile, lift is greatest at the moment of level-off in the cruise. At all other times in the climb, there is a proportion of the thrust vector acting vertically, despite what Capt Bloggs may say

Of course, if you start pulling 2g turns then that's a different matter
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Old 31st Oct 2012, 03:26
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Google suggests that the A380 engines produce a total thrust of 130,000 kg roughly. If the thrust angle was 20 degrees the vertical component would be

130,000 * sin(20) = 44,000Kg
Not even that much, since most planes don't rotate much beyond 10 degrees, and certainly not airliners unless they have cast-iron tails (to handle the ensuing tailstrike). Go ahead and try rotating directly to a 20-degree pitchup on the AH the next time you take off and see what happens....

The typical takeoff and climb angles of all Boeing planes - Bangalore Aviation

More like 10 degrees, which gives 22,100 lbs of "vertical" thrust vector in that calculation.
____________

To me, it just isn't that mysterious or complicated:

First - wings don't just suddenly begin producing lift at V1 or Vr - they are producing some lift at any forward airspeed. The faster you go, the more lift they produce. At just below Vr, they may be producing lift equal to, say, 80% of the aircraft weight, with the remaining 20% keeping the wheels on the ground.

At take-off, you rotate the plane, which increases AoA. Increasing AoA from 0 to 10 degrees increases a generic wing's lift coefficient nearly 3x: File:Lift curve.svg - Wikipedia, the free encyclopedia

3x increase in lift means lift increases quickly from 80% to 240% of aircraft weight - and up you go.

Even a 2 increase in AoA increases lift nearly 50% (and the Buff in that video does rotate about 2 - the nose wheel comes up at about 0:15) - which is enough to increase lift from our assumed 80% of weight, to 120% of weight.

or put even more simply:

Condition 1 - airplane weight > than lift = airplane stays on ground

Condition 2 - airplane weight < than lift = airplane climbs

A plane takes off when the increased AoA of rotation, and subsequent increase in lift, moves the plane from condition 1 to condition 2.

Last edited by pattern_is_full; 31st Oct 2012 at 03:27.
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Old 31st Oct 2012, 21:41
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Lift at 240% of weight would mean pulling 2.4gs at liftoff. I'm only SLF, but I'd think I'd have noticed that.
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Old 31st Oct 2012, 21:54
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The description from Pattern is full is also evidenced by the way wings flex upwards more, the faster that an aircraft goes on its take-off run.

Last edited by Burnie5204; 31st Oct 2012 at 21:57.
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Old 6th Nov 2012, 22:53
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Lift at 240% of weight would mean pulling 2.4gs at liftoff.
Would you care to explain that statement?
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Old 6th Nov 2012, 23:13
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Quote:
Lift at 240% of weight would mean pulling 2.4gs at liftoff.
Would you care to explain that statement?
Shouldn't be a surprise.

N = L / W

We clearly do not pull 2.4 g at take off therefore Pattern's analysis is wrong.

Pb
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